The present invention relates to gas burners, in particular to diffusers for such burner in which the combustion of the gaseous mixture takes place.
Gas burners consist basically of a nozzle through which a fuel gas, such as methane, propane, and the like, under a pressure is injected into a mixing tube, usually a venturi type of tube, where a suitable flow of combustion air or primary air is drawn in, due to the suction generated by the flowing fuel gas.
The primary air, once admixed to the gas in a appropriate proportion therewith, forms a flammable mixture which is directed into a mixing and distribution chamber formed of a box-type body of sheet metal provided with openings called diffusers.
The mixture flows out of the diffusers, of suitable size, at a suitable velocity which varies with flow rate, and is ignited outside the diffusers, either by a pilot flame or suitable igniting devices.
The shape and size of the diffusers is particularly critical to an efficient combustion and to the flame stability under different conditions of operation, as well as for preventing backfiring.
The applicant has developed for this purpose burners with high specific power and low NOx and CO emissions which employ diffusers with pairs of side-by-side grids separated transversely by a screen, from which the inflammable mixture is caused to flow out at a relatively high velocity. The high velocity is a multiple of the flame rate of propagation at full power, when the air-to-gas ratio of the mixture is approximately stoichiometric or higher (0.8&lt;lambda&lt;1.6).
Diffusers of this type, being the subject matter of a patent EP-A 0373157 to the applicant, allow laminar flames or particularly stable flame fronts to be generated which extend across divergent surfaces having their origin located close to the central screen, and spreading out in a variable manner according to the outflow velocity, inversely therewith.
Since the flame develops across two surfaces which are spaced apart from the element or plate whereon the diffusers are formed, the material from which the diffusers are formed is stressed thermally to a negligible extent, is unaffected by ageing due to fatigue and is not subjected to wear or alteration of its structural characteristics.
Thus, the useful life of a burner equipped with this type of diffuser (additionally to its efficiency and specific power) is much longer than the life of conventional burners wherein the flame generated by a flow of a mixture of air and fuel at a velocity slightly higher than the flame propagation rate develops in close proximity to the diffuser with local burner overheating.
Diffusers of this type can operate at variable flow rates, and therefore, under modulated conditions.
However, at reduced flow rates and consequently reduced velocity of the fuel mixture outflow (or with gas used at a higher flame propagation rate than anticipated), the flame tends to approach the diffuser walls causing localized heating of the grid pair and the screen interposed therebetween (similar to that of conventional burners).
In these conditions, the differentiated burner heating, with the temperature surge that may attain 300.degree.-350.degree. C., causes local internal expansions and stresses which fatigue the material and result ultimate failure of the same.